Another way to get her comfortable in the tummy position is to carry her around with her tummy rested on the inside of your forearm.  It isn’t as rigorous of practice compared to when she is doing tummy time on a flat surface, but it is a great way to transition into that position eventually.  Make sure to do both arms so she gets to practice balance on both sides.  Aim to carry her in this position for at least 15 min.  This will count as 1 round of your 4 daily sessions of tummy time.  Feel free to repeat this exercise.  Also, It’s a good bicep workout for the person carrying.